Background Art
In the prior art (IBM Journal of Research and Development, Vol. 26, No. 2, March 1982, pp. 136-144), a laser beam impinging on an electrode is used to enhance local electroplating or etching rates by several orders of magnitude. With the aid of the laser it is possible to produce very highly localized electrodeless plating at high deposition rates to greatly enhance and localize the typical metal exchange (immersion) plating reactions and to obtain a thermo-battery driven reaction with simple aqueous solutions. Since laser beams can be readily focused to micron sized dimensions and scanned over sizeable areas, laser enhancement makes it possible to plate and etch arbitrary patterns without the use of masks.
Exchange plating occurs when a less noble metal surface is immersed in an electrolyte containing a more noble metal. In general, the process is very temperature-dependent with increased exchange plating rates occurring at higher temperatures. By locally heating the surface with a laser beam, it is possible to establish a temperature difference wherein the laser illuminated (heated) area acts as a cathode establishing a thermal potential difference with the unilluminated (cooler) portion of the surface. This technique has also been demonstrated where both the metal and the element in solution are of the same nobility (IBM Technical Disclosure Bulletin, Vol. 24, No. 1A, June 1981, p. 249), the energy required for thermal plating being provided by the thermo-battery effect.
In view of the highly localized nature of laser-enhanced exchange plating and the fact that no mask is required, it would be advantageous to use such plating methods for the repair of discontinuities in conductors on integrated circuits. More particularly, it would be advantageous to utilize laser-enhanced exchange plating in the repair of integrated circuits which have been covered by a passivation layer.
Laser enhanced exchange plating has been suggested as a means of repairing integrated circuits. See, for example, U.S. Pat. No. 4,349,583 to Kulynych et al. assigned to the present assignee. This patent is hereby incorporated herein by reference. Kulynych describes a method for high resolution maskless immersion, exchange or like plating to a substrate covered by a conductive base where the plating rate is increased through the use of a laser. U.S. Pat. No. 4,239,789 to Blum et al. assigned to the assignee of the present invention, also describes electrodeless plating onto a pre-metallized substrate.
As an alternative to the continuous metallic base plate of Kulynych et al., a non-continuous base layer may be provided (as in German Pat. Publication DE 31 39 168 A1), by depositing a PdCl.sub.2 solution on the surface of the substrate to "activate" the substrate. The desired region may then be plated by the maskless exchange plating method described previously.
An alternative method of circuit repair via laser-enhanced exchange plating is described in Research Disclosure No. 26123 (No. 261 published January 1986, by Kenneth Mason Publications Ltd., England). This abstract describes a means of preventing the undesirable peripheral etching which normally accompanies laser-enhanced exchange plating. Etching is prevented by covering all exposed portions of the circuit being repaired with a "shim stock" which is in electrical contact with the circuit line and provides the ions necessary for exchange plating. The exchange plating therefore etches the shim rather than the circuit line. This means of protecting the circuit line has the disadvantage of requiring the use of a shim which substantially covers the exposed portion of the line to be plated to.
Plating techniques have also been described wherein a metallic conductor was plated directly to a non-metallic substrate by immersing the substrate in a plating solution and heating the substrate in the region where deposition is desired. This heating could be accomplished in any number of ways, including exposure to a high intensity light through a mask or the use of a laser to selectively irradiate the surface of the substrate, causing the metallic plating material to deposit out in the irradiated region.
U.S. Pat. No. 4,578,157 to Halliwell et al. describes a technique for using a pulsed laser to plate directly to a nonmetallic GaAs surface. In this patent, no conducting layer is used as a base for plating. U.S. Pat. No. 4,578,155 to Halliwell et al. extends this concept to the plating of a polymeric substrate.
U.S. Pat. No. 4,359,485 to Donnelly et al. describes an alternative method for plating a metal layer onto the surface of a group III-V compound semiconductor by placing the surface in contact with a metal containing solution and directing laser radiation through the solution. This maskless deposition technique is used to form elongated metal conductors on the substrate.
While all of these prior art techniques address the problem of maskless plating of conductors onto semiconductor surfaces, none of the references discussed to this point address the creation of a conductive layer within the surface of the substrate as a base for depositing plating and subsequently removing that conductive layer in the regions where selective plating was not deposited. Nor do they describe or suggest a maskless means of accessing and repairing discontinuities in conductors disposed on a substrate and covered by a passivation layer.
Other publications have disclosed the deposition of a conductor on a non-metallic structure which has been prepared by exposure to laser irradiation in the region where plating is desired. Japanese Patent Publication 61-104083 describes a method of preparing a substrate to be plated by first exposing the region to be plated to a laser which prepares the plating surface. This preparation is followed by electrodeless plating of the substrate to the region irradiated by the laser. Japanese Patent Publication 61-96097 describes a method of plating wherein the substrate is exposed to electrolyte flashing and laser beam radiation simultaneously, the laser beam being passed coaxially through a nozzle which also feeds electrolyte to the substrate surface.
U.S. Pat. No. 4,691,091 to Lyons et al. extends this concept to producing electrically conductive paths in a polymeric substrate by laser writing, (i.e., by tracing the desired paths on the substrate using a laser beam). The resulting paths comprise electrically conductive carbon produced by thermal decomposition of the substrate surface material. Lyons et al. point out that these conductive patterns may be plated with a metallic conductor. U.S. Pat. No. 4,663,826 to Baeuerle describes an alternative method for generating an area of increased conductivity on the surface of a body of a dielectric material wherein that material is exposed to laser radiation for a selected period of time within a reducing atmosphere. This results in a "tempering or sintering" of the material, causing an oxygen depletion within the material and resulting in an n-type semiconductor conductivity region which provides a conductive region to which metal may be plated by electroplating or other conventional plating techniques. Thus in these references the area to be plated is defined by the laser damaged regions.
In certain instances, it is necessary to remove a portion of a passivation or other layer covering a substrate in order to either access or deposit a conductor on that substrate. An example of the latter situation is the removal of photoresist in a specified region prior to deposition of a conductive run. An example of the former situation is where a substrate has been passivated--that is, covered with a passivating material to protect the substrate and the conductors thereon--before it is discovered that there are discontinuities in the conductors disposed on the substrate, it may become necessary to remove a portion of the passivation to repair discontinuities in the conductor. These two situations present entirely different problems. In the first situation, a mask determines the regions of the photoresist to be removed and this mask may be used repetitively for any number of substrates. In the second situation, the discontinuities will appear in unpredictable locations and each discontinuity will have to be individually accessed in order to repair the damaged or missing conductor. Thus, it is impossible to design or to manufacture a mask specifically designed to repair all discontinuities, especially where the discontinuities are being repaired in newly manufactured semiconductor chips. The ability to easily access and repair discontinuities in large scale integrated circuits is extremely important since it facilitates salvaging of integrated circuits which were previously discarded as being unusable.
U.S. Pat. No. 4,574,095 to Baum et al. and assigned to the assignee of the present invention describes a method of selectively depositing copper by first selectively depositing palladium seeds and subsequently irradiating a palladium compound with light through a mask. A substrate is first covered with photoresist or polymer. The coated substrate is selectively irradiated, for example, through a mask, with a pulsed excimer laser, and the polymer in the irradiated area is removed. When the substrate is exposed to the vapors of the palladium containing complex, deposition of a metal film occurs at the exposed portion. The metal film acts as a seed for plating of copper. In this way, the circuitry is imbedded in a polymeric film, the surfaces of which are level. The circuitry is thus protected from mechanical damage. The ablative etching and the deposition of the metal seed can take place either simultaneously or as separate steps. While this reference refers to the ablation of a polymer coating prior to deposition of a metal seed layer and plating of a conductor, it is accomplished by using a predefined mask to define the regions to be ablated.
None of the cited references describes a means for locally accessing and repairing damaged conductors which have been subsequently covered by an insulating or passivation layer. Since most defects in semiconductor ICs are not discovered until after the IC is manufactured, the repair of semiconductor IC runs which are not accessible from the surface is a critical need. It would be extremely useful and cost effective to find a way of quickly and accurately repairing small breaks in IC conductor lines. In addition, it would be advantageous to accomplish such repairs without the necessity of a mask step. It would also be advantageous to accomplish such repairs by thermo-plating the region between the broken lines. It would further be advantageous to accomplish repair of such IC conductor lines without the necessity of depositing a conductive layer on the surface of the semiconductor substrate prior to thermo-deposition of the repair bridge.